A Fast Sphere Decoding Algorithm for Space-Frequency Block Codes
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A Fast Sphere Decoding Algorithm for Space-Frequency Block Codes Zoltan Safar,1, 2 Weifeng Su,1, 3 and K. J. Ray Liu1 1 Department
of Electrical and Computer Engineering, A. James Clark School of Engineering, and Institute for Systems Research, University of Maryland, College Park, MD 20742, USA 2 Modem System Design, Technology Platforms, Nokia, Copenhagen 1790, Denmark 3 Department of Electrical Engineering, School of Engineering and Applied Sciences, State University of New York, Buffalo, NY 14260, USA Received 10 November 2004; Revised 8 August 2005; Accepted 22 August 2005 The recently proposed space-frequency-coded MIMO-OFDM systems have promised considerable performance improvement over single-antenna systems. However, in order to make multiantenna OFDM systems an attractive choice for practical applications, implementation issues such as decoding complexity must be addressed successfully. In this paper, we propose a computationally efficient decoding algorithm for space-frequency block codes. The central part of the algorithm is a modulation-independent sphere decoding framework formulated in the complex domain. We develop three decoding approaches: a modulationindependent approach applicable to any memoryless modulation method, a QAM-specific and a PSK-specific fast decoding algorithm performing nearest-neighbor signal point search. The computational complexity of the algorithms is investigated via both analysis and simulation. The simulation results demonstrate that the proposed algorithm can significantly reduce the decoding complexity. We observe up to 75% reduction in the required FLOP count per code block compared to previously existing methods without noticeable performance degradation. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
1.
INTRODUCTION
Multiple-input multiple-output (MIMO) systems employing multiple transmit and receive antennas have the potential to play a significant role in the development of future wireless communication systems. By exploiting the larger number of propagation paths between the transmitter and the receiver, the adverse effects of the channel fading can be significantly reduced. To take advantage of both the MIMO systems and the OFDM modulation, MIMO-OFDM systems have been proposed recently [1–6], where space-frequency (SF) coding is applied as the channel code. However, before such a system becomes an attractive choice for practical applications, implementation issues such as decoding complexity must be addressed successfully. Computationally efficient decoding algorithms have only been proposed for decoding space-time (ST) block codes in quasistatic, flat (frequency nonselective) fading environment [7, 8]. For ST block codes transmitted over temporally evolving channels and for SF block-coded MIMO-OFDM systems, where the channel changes along the frequency axis, lowcomplexity decoding algorithms still do not exist in the literature.
The sphere decoding algorithm was introduced in [9] assuming a single-antenna, real-valued fading channel model. Lat
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